THURSDAY, Dec. 4, 2014 (HealthDay News) -- Although low-dose aspirin may curb the risks of heart disease and colon cancer, the downsides appear to outweigh the benefits for many women, a new large study suggests.

For women younger than 65, researchers found taking low-dose aspirin for years lowered the risks of heart attack, stroke and colon cancer by a small amount.

But they also found that the benefit was countered by an increase in the risk of major gastrointestinal bleeding -- serious enough to land a woman in the hospital.

The picture looked better for women age 65 and up. Aspirin boosted their risk of bleeding, too -- but the benefits against heart disease and colon cancer were bigger, researchers reported.

The study appeared online on Dec. 4 in the journal Heart.

Many people have heard that low-dose aspirin is good for the heart, and may feel like it's a good idea to take some every day. But the reality is much more complicated, said Dr. John Erwin, a cardiologist at Scott & White Memorial Hospital in Temple, Texas.

"There's no question that aspirin can be a lifesaver for people who've already had a heart attack," said Erwin, who was not involved in the new study.

For those people, he explained, a daily aspirin -- if recommended by a doctor -- can help prevent a repeat heart attack or other cardiovascular complications.

Things get murky, however, when it comes to preventing a first-time heart attack, or any other disease -- what doctors call "primary prevention." It's still unclear which people stand to gain a benefit that will outweigh the risks of aspirin -- namely, ulcers, stomach bleeding, or even bleeding in the brain.

"It's been a huge conundrum for us over the years," Erwin said. "When it comes to primary prevention, there are relatively few patients who will get a big benefit. And there's always the risk of harm."

The idea of using low-dose aspirin for primary prevention has gained some steam from research showing it may also cut the risk of colon cancer. But many medical experts still urge caution.

The American Cancer Society advises people against popping the drug solely to ward off cancer. Similarly, the American Heart Association recommends only that people at "high risk" of a heart attack consider taking aspirin.

Meanwhile, the U.S. Food and Drug Administration recently came out against using aspirin for preventing first-time heart attacks and strokes -- citing the potential harms.

According to Nancy Cook, one of the researchers on the new study, the risks may not be worth it for most women younger than 65.

"I probably wouldn't take aspirin unless I had a very high risk of either cardiovascular disease or colon cancer," said Cook, a professor of medicine at Harvard Medical School and Brigham & Women's Hospital in Boston.

The findings are based on a clinical trial of nearly 28,000 women who were largely healthy and relatively young -- about 55 years old, on average, at the start of the study. They were randomly assigned to take low-dose (100 mg) aspirin or placebo pills every other day.

Over the next 15 years, about 11 percent of the women either developed cancer, suffered a heart attack or stroke, or died of cardiovascular causes, according to the study. Women who'd taken aspirin saw a small decrease in their odds of cardiovascular trouble or colon cancer -- but at the expense of an increase in the risk of gastrointestinal bleeding, the researchers found.

That expense was clear among women younger than 65.

The researchers estimate that for every 133 women on aspirin for 15 years, one would suffer a major gastrointestinal bleeding episode -- serious enough to warrant a hospital stay. And one out of 29 women would have less serious problems: a stomach ulcer or slight bleeding in the digestive tract.

By comparison, 709 women would have to take aspirin to prevent one case of colon cancer; and 371 would have to regularly take the drug to ward off one cardiovascular complication, according to the study.

The picture changed, however, as women grew older. Among those age 65 and up, 29 would need to take aspirin, long-term, to prevent one case of cardiovascular disease or cancer.

But she and Erwin both stressed that the decision ultimately depends on a woman's personal risk factors.

Erwin pointed out that the women in this study were generally healthy to start. "You wouldn't expect to see much benefit in a healthy group," he noted.

For women with major risk factors for heart trouble -- such as diabetes or high blood pressure aspirin might do more good, Erwin said. Similarly, women with risk factors for colon cancer, like a strong family history, might gain more benefit, according to Cook.

"What it comes down to," Erwin said, "is that people need to have a careful discussion of the potential benefits and risks with a doctor they trust."